Blends of thermoplastic resins are known and commercially available. A thermoplastic resin is often selected to enhance or supplement the properties of the other blend components (see, for instance, U.S. Pat. Nos. 4,317,890; 4,342,847 and 5,194,494). Commercial molding compositions containing TPU and ABS have long been available. The art is noted to include U.S. Pat. No. 4,251,642 which disclosed a blend of polyurethane with a block copolymer of styrene and certain conjugated diolefins and a graft copolymer of styrene, acrylonitrile and butadiene (ABS). Also relevant is U.S. Pat. No. 3,049,505 which disclosed a blend containing ABS and thermoplastic polyurethane (TPU) characterized in its improved tear strength. U.S. Pat. No. 5,614,589 disclosed a transparent composition containing methacrylate-acrylonitrile-butadiene-styrene (MABS), a specific TPU and a compatibilizer in the form of emulsion ABS. U.S. Pat. No. 5,216,062 disclosed a composition containing TPU, a grafted rubber (such as ABS or ASA) and a copolymer of alpha methyl styrene and (meth)acrylonitrile.
For many applications, it is desirable to have a transparent plastic composition. Transparency is particularly desirable for packaging materials, vessels, and containers to permit visual inspection of the contents. Transparency in the present context is understood as a property enabling light transmission (herein LT value) of at least 40% as measured according to ASTM D-1003. In order to achieve this level of transparency, one skilled in the art would select a transparent TPU and a transparent copolymer of styrene and acrylonitrile (SAN) that are (i) reasonably compatible to achieve a uniform dispersion and (ii) virtually identical in terms of their refractive indices. One skilled in the art would also know that identifying the SAN and TPU components meeting these criteria is not a trivial matter because TPU and SAN are generally incompatible. Moreover, the difference in their refractive indices render opaque blends of these components. This is to say that despite the transparency of the individual components, the resulting blend is typically opaque or the molded articles are opaque. To achieve and maintain the dispersion through processing to a final molded article, it is necessary to improve the interaction that results during the mixing process to limit phase separation. If the mixing is insufficient, the problem manifests itself in the form of opacity, non-uniform melt flow, and unreasonably large variations in physical properties.
Blends of thermoplastic polyurethanes with other thermoplastics suffer from various processing problems including non-uniformity of melt flow. To improve the processing of such blends, Carter (U.S. Pat. No. 4,179,479) disclosed adding a small amount of polymethyl methacrylate (PMMA). The PMMA appears to have promoted the dispersion, surface gloss and processing of the compositions containing TPU and ABS. Although Carter exemplifies clear pellets of compositions containing TPU and ABS, PMMA is added in relatively small amounts.
The preparation of blends of TPU with SAN have been attempted previously; these had unpredictable physical and optical properties. Zerjal, et al. (Structure-Property Relationships in Polyurethane-Poly(styrene-co-acrylonitrile) Blends), Journal of Applied Polymer Science, Volume 50, pp. 719-727, (1993) reported that the blending of TPU and SAN results in many property variations in the finished blend. Paul and Newman (Polymer Blends, Academic Press, New York, 1978, Volume 1) attributed this to the unmixing of TPU/SAN solutions over all composition ranges and concluded that these blends are immiscible. The challenge to create stable blends of SAN and TPU was to form a blend that remained mixed under a variety of processing/molding conditions and remains transparent. One skilled in the art could recognize that the miscibility of the SAN and TPU components is dependent on the composition of the individual components. Although individual compositions of SAN and TPU may be transparent under limited processing conditions, they rarely remain transparent without special processing. The present invention describes the use of an acrylate polymer, preferably PMMA, that acts as a compatibilizer for SAN and TPU to maintain transparency through the forming of the finished part.